WASHINGTON -- Massive peat bogs in Siberia and elsewhere may have helped spur global warming at the end of the last ice age some 12,000 years ago, scientists reported Thursday.

The ice was already melting when the bogs formed, but the fact that they emitted the greenhouse gas methane accelerated the warming trend, said Glen MacDonald, a climate change expert at the University of California Los Angeles.

But this does not take humans off the hook, given that the amount of methane sent into the atmosphere since the start of the Industrial Revolution is far higher than what occurred naturally from decomposing material in the old bogs, he said.

"The amount of methane that we have added to the atmosphere is even more extreme than the rate of this change that happened at end of the last ice age," MacDonald, the study's lead author, said in a telephone interview. "Over the last 200 years we have more than doubled the amount of methane in the air."

At the close of the last ice age, some 12,000 years ago, the bogs "really turned on," MacDonald said, and caused a rise in the atmosphere's methane level from 450 parts per billion by volume to 750 parts per billion.

But from the year 1750 until the present, he said, methane levels went from 750 parts per billion to 1,700 parts per billion.

NOT A NATURAL INCREASE

"About 60 percent of the methane going into the atmosphere is anthropogenic," or human-caused, MacDonald said. "Natural sources today aren't really capable of producing the spike that we're seeing. Methane has reached levels that are unprecedented."

Methane, known to scientists as CH4 and colloquially as swamp gas, is an odorless, colorless gas that contributes to the greenhouse effect, in which the sun's heat is trapped in the lower atmosphere, warming Earth.

It occurs widely in nature when bacteria help decompose plant and animal material, without the benefit of oxygen. This happens in big bogs, which are made up of a thick layer of incompletely rotted dead organic matter beneath a layer of living vegetation.

By taking core samples of peat bogs in Siberia, and adding that data to samples of other peat bogs, the scientists determined that these peatlands formed a bit before the end of the last ice age, but really grew explosively as it ended.

Methane is said to be up to 23 times more potent as a greenhouse gas than carbon dioxide, so knowing a potential source of methane is important, the researchers said.

This research, by scientists from the University of California Los Angeles and the Russian Academy of Sciences, was published in the current edition of the journal Science.